There are two possible behaviors you might be intending:
- Rename one or more existing keywords by doing search-and-replace on their names. The assignment of keywords to photos doesn't change, just the names of the keywords. I can see practical (but infrequent) use cases for that, e.g. correcting a misspelling that has been repeated in many keywords' names. Though the SDK would support this, I don't know of any plugins providing this functionality.
- Do search and replace on the IPTC keywords field, treating it as a text string (the same string that appears in the Keywording panel). This would be logically equivalent to editing what appears in the Keywording panel. This would change which keywords are assigned to photos, possibly adding new keywords, but it wouldn't rename existing keywords. It's possible to accomplish the same end with current features, by first filtering photos for those with specific keywords, putting those photos in the Quick Collection, then editing the keywords assigned to them using the Keywording panel.